1. Field of the Invention
The present invention relates generally to data storage subsystems, and more particularly to methods of retrieving data from a data storage subsystem.
2. Description of Related Art
U.S. patent application Ser. No. 09/274,789, filed Mar. 23, 1999, entitled ASYNCHRONOUS SHADOWING IN DATA STORAGE SUBSYSTEM, applicant, Christopher J. West and U.S. patent application filed concurrently herewith, entitled STORING AND RETRIEVING DATA ON TAPE BACKUP SYSTEM LOCATED AT REMOTE STORAGE SYSTEM SITE, applicant, Christopher J. West, and David G. Beal are specifically incorporated herein by reference for all that they disclose and teach.
In today's competitive business environment, a business must diligently protect its assets, including the data stored within its computer network. Safeguarding corporate data, including engineering designs, pricing information, corporate procedures, customer and vendor lists, and order information, is crucial to the long-term success of a business. Threats to corporate data include hardware failure, physical plant disasters (e.g., building fires, earthquakes, tornadoes, and hurricanes), theft, and industrial sabotage. Protecting corporate data through backup software helps alleviate the downside of these threats and therefore has become an essential part of managing the network environment.
Data storage management methods in modern computer networks tend to vary according to different data characteristics and management issues, such as the amount of the data, cost, performance and fault tolerance. For example, one method of protecting business data is to periodically copy (or “backup”) the data to an archive tape for storage at a remote location. Other, more sophisticated solutions involve real-time archival of large amounts of data. Such systems may be generally referred to as “remote copy system”, wherein data is recorded fairly contemporaneously to a primary storage medium and a secondary storage medium at another location. Typically, the primary storage medium creates a primary “volume” which represents a set of data information accessible by the primary host application. The secondary storage medium is coupled to the primary storage medium and creates a secondary volume. The primary and secondary volumes form a pair wherein the secondary volume contains the copy of the primary volume.
One implementation of remote copy, called “Peer-to-Peer Remote Copy” or PPRC, provides a synchronous copy, in that an application host system's write operations are propagated to both the primary storage medium and the secondary storage medium at the remote site before the application host system receives an acknowledgment from the storage system indicating that the write operation has completed. In PPRC, the secondary volume is always an exact copy of the primary volume when the pair is duplex.
Another implementation of remote copy, called “Extended Remote Copy” or XRC, provides an “asynchronous copy”, in that a write access by an application host system is signaled as complete when the write operation to the primary storage medium completes. The remote copy to the secondary storage medium is performed by a System Data Mover (SDM), a software component executing on a host system (i.e., on the application or primary host system, on the recovery or secondary host system, or on a third host system) having connectivity to both the primary volume and the secondary volume. The SDM periodically copies the data from the primary volume to the secondary volume in the order which the data was originally written to the primary volume.
Periodically copying data from the primary volume to the secondary volume creates a “point-in-time” copy of the data. That is, the information located on the secondary volume is a reflection of the data that was on the primary volume at the time the data was transferred. At a later time, the information on the primary volume may change, and until those changes are propagated to the secondary volume, the point-in-time copy is not identical to the primary volume. In contrast, when two volumes are in duplex under the synchronous PPRC scheme, changes are propagated to the secondary volume as the changes happen, and therefore, the secondary volume is identical to the primary volume at all times.
Yet another implementation of remote copy involves the asynchronous “shadowing” of data between a primary storage controller and a secondary storage controller wherein a snap volume is created by a primary storage controller, the snap volume is used in place of the primary volume to facilitate remote copying instead of the primary volume. Asynchronous shadowing of data provides benefits over the synchronous PPRC in that the host computer is not required to wait for completion of the remote copy process before accessing the primary volume. Additionally, asynchronous shadowing allows the host computer to access the primary volume more readily than in the XRC since the primary volume is not used in copying data to the remote location. The details of asynchronous shadowing and its benefits are described in more detail in the co-pending application referenced above.
With any one of these remote copy implementations, their primary purpose is to provide a copy in case of an emergency or, more particularly, to provide disaster relief wherein the entire computer subsystem or disk array fails or is lost due to a tragic event. As such, the focus of most remote copy systems has been to ensure that the data is properly backed up.
In some situations however, a user of a host computer system desires recovery of backed up data even when the primary storage system and primary host computer are operating properly. In essence, there are times when the data on the primary storage system is not available, for various reasons, such as for example, human error may cause a file to be accidentally deleted or modified. In such a case, the user wants the backup copy of the file or data, if such a backup exists.
If synchronous PPRC is used, then the process of retrieving backup data initially depends on whether a tape backup copy was made, typically by a wholly different tape backup system. That is, since write/modify operations propagate to the secondary system in the PPRC scheme, the secondary system is changed as soon as the primary system is changed. Thus, human errors corrupt the secondary system in the same manner as the errors corrupt the primary system and recovery from the secondary is generally not possible.
Asynchronous remote copy methods and systems on the other hand, while potentially providing a point in time copy, typically do not provide for the transfer of data back to the primary volume from the secondary volume. Recovery operations are typically achieved by copying the data onto a tape medium and transferring or shipping the tape medium back to the primary site. Using the tape medium is labor intensive and time consuming.
It is with respect to these and other considerations that the present invention has been made.